This research is a long-term study of rhesus monkey populations in north central India begun in 1959. Accurate censuses of rhesus groups in Aligarh district, Uttar Pradesh, are conducted three times a year to obtain data on group sizes, sex and age ratios, birth rates, mortality rates, population trends, annual turnover, and interactions between people and monkeys. In 1979-80, re-surveys of rhesus populations along roadsides, canal banks, in villages, and in forest areas are being undertaken to compare changes in these populatons over the past twenty years.